Mileage Moment of Truth: We Put 40 Mpg Claims to the Test

The 2012 Hyundai Elantra and Ford Focus SFE are among 20-plus cars that now claim 40 mpg highway. But given the peculiar way in which the EPA calculates its fuel economy estimates, do those mileage numbers truly reflect real-world driving? We tested the Elantra and Focus on the streets and roads of Michigan to find out, with some surprising results.

Gas prices remain stubbornly high, and the economy is still lagging. Carmakers are responding to consumer demand for smaller, more efficient cars (and to stringent government fuel-economy standards) with a raft of vehicles claiming an astonishing 40-mpg highway rating. When a gallon of gas first topped $4 in 2008, only the Toyota Prius and the Honda Civic Hybrid got over 40 mpg on the highway; now there are more than 20 cars able to achieve that kind of mileage. Over the past 12 months we've tested a variety of 40-mpg cars and have regularly missed hitting the window-sticker figures. Others have cried foul as well. Last December, a nonprofit group called Consumer Watchdog sent a ­letter to the Environmental Protection Agency suggesting that the Hyundai Elantra's rated fuel economy (29 city and 40 highway) was erroneously high. Perhaps, we wondered, the auto­makers had found a way to game the EPA's test. Then again, we're also ­keenly aware that driving behavior has a pronounced effect on fuel economy. So which is it? Are the cars overstating their ­efficiency, or are aggressive drivers like us distorting the results with our lead feet? We obtained a 2012 Hyundai Elantra and another 40-mpg car, the Ford Focus SFE, and did our own fuel-economy test. The results proved surprising.

The Test

It's hard to replicate the real world in a lab. But since 1975, the EPA has published test procedures that carmakers follow to determine the city and highway fuel-economy figures found on new-car window stickers (see "Behind the EPA Numbers" below). Over the years, these tests—which are performed on dynamometers—and their corresponding calculations have been refined to better approximate real-world results. The latest update was in 2008, when the EPA included air-conditioning usage and its correction factors to bring down the window-sticker numbers.

For our testing, we specifically didn't want to replicate the way the EPA tests cars. This wasn't just for the sake of being contrarian, but because we wanted­ to test the cars the way any reader might be able to, which meant driving the cars on the road. We were mainly after the highway-fuel-economy figure, but we also did a city-driving loop. The highway route took a nice big lap around the city of Detroit, twice. It was a lot of seat time, but at about 133 miles, a good distance to get a real-world average. We ran the highway test at both 55 mph and 70 mph, and to avoid traffic variables, we drove in the middle of the day, when fewer cars were on the road. The city course was a meandering in-town 64.5-mile route up into the northern suburbs of Rochester Hills and back down along the riverfront, then up historic Woodward Avenue.

Accurately measuring the miles driven and the gallons consumed is tricky. In-car odometers often don't match brand to brand, so we relied on the $570 PerformanceBox from Racelogic and its more accurate GPS-based distance measurement (we also used this to average our speed). Tracking the gallons used is similarly prickly. The procedure most people use is to top off the fuel tank at every gas stop and ­simply record the miles. According to Hyundai, however, variations in ambient conditions can result in one fill-up being as much as a half-gallon less than the next at the same pump. So to find out exactly­ how much fuel was used, we weighed the cars precisely before and after driving. Each gallon of standard E10 gasoline weighed in at 6.4 pounds, so if we drove 80 miles on the highway and the cars achieved 40 mpg, then they should be 12.8 pounds lighter than at the start—a simple but effective method.

Results

Counter to our original hypothesis, both cars demonstrated significantly­ better fuel economy than advertised. Cruising along at 55 mph on the highway, our cars easily cleared 40 mpg and, astonishingly, approached 50. At higher speeds, with greater aerodynamic drag, the cars were still very efficient. They didn't quite get 40 mpg, but they were close. City results were equally­ impressive, with each into the mid-30s. Bear in mind that we made no effort to be overly frugal—no drafting, no excessive coasting—and we made a point to keep up with traffic. Sure, we were a little light with the pedal, but slowpokes we were not.

What, then, should we make of our own previous fuel-economy tests and the cries of Consumer Watchdog? To put it simply, your results will vary, and that is why the window-sticker figures are called estimates. For our test, we simply concentrated a little more on thrifty driving than usual, and it was 40 F outside, so we didn't use the air conditioning. There will never be a lab test that can cover all environmental variables or account for how differently we all drive. But these two cars demonstrate that with very little behavior modification, 40 mpg is quite a realistic figure. Not only is it easy to achieve, it's easy to surpass, even under less than ideal conditions. If you choose a car with a high-economy claim and drive within reason, you should be able to match those window-sticker figures. Considering that these cars are also decent performers on the road, the benefit of this high-efficiency engineering really goes to consumers, who are apparently getting more than they've bargained for.

The Fuel-Economy Equation

Many factors determine efficiency—not the least of which is how aggressive you are on the throttle. Still, things like vehicle size, weight, and shape; road friction; outside temperature; aerodynamic drag; and losses in the powertrain all play important roles. Collectively, these factors are known as a vehicle's road load. We've created a simplified version of the road-load equation. Notice how velocity is squared in the equation, meaning each additional mph has a greater impact on your mpg.

Behind the EPA Numbers

Getting the window-sticker fuel-economy estimates starts with a car strapped on a dynamometer, which positions the car's drivewheels on a giant computer-controlled rolling pin. A technician operates the car following prescribed acceleration and speed curves. There are five different test procedures: city cycle, highway cycle, air conditioning on, high-speed driving, and cold starting for carbon monoxide emissions. All currently use 100 percent gasoline, something consumers generally cannot buy because 10 percent ethanol mix is what most pumps dispense. Flow meters measure fuel consumed, and the exhaust gas is collected for emissions measurements.

The tests are surprisingly slow and short—the longest is 11 miles, and the highest average speed is 48 mph. In this scenario, cars get much higher fuel economy than what's advertised to consumers. To adjust that perfor­mance toward real-world figures, the EPA applies mathematical formulas to determine the final numbers on the window sticker. Further complicating matters is that for the Corporate Average Fuel Economy (CAFE), the government uses the uncorrected data. For the CAFE, our Elantra gets 56.5 mpg highway, but after correction the window sticker says 40 mpg. Complicated?

We think so, too.

PM Mileage Test at a Glance

We took two closely matched, high-mileage vehicles on multiple test loops to see if they could hit their EPA window-sticker numbers on real roads. We found that if you're willing to ease the pedal off the metal, these sippers outperform the ratings.

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